Space based laser weapons and other laser systems based in space are required to focus the laser beams on targets at great distances. To accomplish this requires focusing optics with very large output aperture. For example, the diameter of the exit aperture of the antenna or telescope can be determined from the range and desired laser spot size to be produced at the target. This diameter is given by: ##EQU1## where .lambda. is the wavelength of the laser being used, e.sub.T is the diameter of the focal spot at the target (or at the receiver if this happens to be a relay mirror), and R is the range or distance to the target. In some applications the exit aperture diameter may have to be 20 meters or larger and will be required to have near diffraction limited performance.
Such large good quality optics did not previously exist, and if one could be made by standard techniques then it would weigh so much that it could never be placed into space. For example, the mirror of the 200 inch (about 5 meters) telescope located at Mount Polomar, Colo., required more than four years to manufacture and weights much too much for current boosters. Also for defense applications, many very large optics are required, and for other applications more than one will be required. Because of this, much work has been done, and is being done, to develop lightweight large optics that might be made in parts and later erected in space. Although some progress has been made, these devices are still heavy, costly, much too small, and require too long to produce. Also, good optical quality is difficult to obtain and more difficult to maintain over these large diameters.
The gas dynamic lens used in application Ser. No. 510,709 overcomes many of these problems in that it can be erected in space when needed and can be repeatedly erected when necessary. It is lightweight, has the required large aperture and the optical quality problem is relaxed by the use of a gradually changing index of refraction rather than the discontinuous jump which the index of refraction suffers when solid optics are used (the solid-air interface discontinuity). This lens also has the advantage that it may be used with light from known high energy lasers.
However, it has the disadvantage of a fixed focal length and the disadvantage that it requires large volumes of gas if it is to be used for appreciably long times or for many times.
Therefore, it is an object of this invention to provide a large gas dynamic lens which can be used in outer space applications and which has the feature that its focal length can be varied so that the system may be focused on targets at different ranges.
Another object of this invention is to provide a large static gas lens which can have the focus thereof varied and which may be used with both cw and pulsed laser.
Still another object of this invention is to provide a static gas lens which does not require a large gas supply system and therefore a lens that can be used over a long period of time with a relatively small gas generator supply for prolonged use of the device.
Other objects and advantages of this invention will be obvious to those skilled in this art.